1. Field of the Invention
The present invention relates to a technology to measure a voltage fluctuation waveform near a particular functional circuit in operation, which circuit is included in a semiconductor integrated circuit (e.g., an LSI (Large Scale Integration) serving as a microprocessor) of CMOS (Complementary Metal Oxided Semiconductor).
2. Description of the Related Art
Recent LSI development has gone through increase in consumption electric power in accordance with speeding up the performance, so that the art demands realization of comparison and verification between results of voltage fluctuation waveforms obtained by a simulation performed during wiring design and by actual measurement performed on a final product LSI.
In order to measure a voltage fluctuation waveform immediately near an circuit in operation in an LSI, a conventional method adopts a destructive inspection using FIB (Focused Ion Beam) or a disclosure in Japanese Patent Application KOKAI (Laid-Open) No. 2001-59855 in which an observation terminal is installed to directly measure a power-source voltage and a voltage fluctuation waveform is measured via the observation terminal.
The former destructive inspection method (the former measurement method) treats a wiring layer unit 101 of an LSI 100 with FIB as shown in FIG. 15 and thereby forms a FIB hole 104 that reaches the bottom wiring layer (a power-source wiring) 101-1 located near a measurement-object operating circuit 102 from the top surface (the surface having a pad 103) of the LSI 100. As a result, the bottom wiring layer 101-1 is exposed as a metal exposure layer 104a. After that, the metal exposure layer 104a is irradiated with EB (electron beams) and a voltage fluctuation waveform of the operating circuit 102 is measured on the basis of secondary electrons bounce off the metal exposure layer 104a. 
The wiring layer unit 101 of the LSI 100 shown in FIG. 15 includes six metal wiring layers 101-1 through 101-6. X-direction wiring layers 101-1, 101-3 and 101-5 and y-direction wiring layers 101-2, 101-4 and 101-6 are alternatively piled up from the bottom to form the wiring layer unit 101.
On the other hand, the latter method (the latter measurement method) using an observation terminal previously designs a wiring that connects the position (the wiring of the bottom layer) of a measurement-object circuit at the bottom layer of an LSI and the position of the observation terminal (the pad) on the top layer of the LSI. With this design, the voltage close to the measurement-object circuit is directly introduced outside the LSI through the observation terminal and a voltage fluctuation waveform is observed.
The recent LSI development is inclined to increase the number of wiring layers and decrease an operating voltage.
An LSI having an increased number of wiring layers makes the former measurement method difficult to expose the bottom layer thereof. For example, the LSI 100 shown in FIG. 15 has six wiring layers. But, in an LSI 200 shown in FIG. 16 having ten wiring layers, it is extremely difficult to expose the desired bottom power-source wiring (the wiring layer 201-1), located on the bottom layer, that supplies the operating circuit 202 by forming FIB hole 204 keeping out signal lines.
In the LSI 200 shown in FIG. 16, a wiring layer unit 201 includes ten metal wiring layers 201-1 through 201-10. Treating the wiring layer unit 201 of the LSI 200 with FIB is to form an FIB hole 204 that reaches the bottom wiring layer (power-source wiring) 201-1 near a measurement-object operating circuit 202 from the top surface (a surface with a pad 203) of the LSI 200. When the LSI 200 has an increased number of wiring layers, the diameter of the opening of the FIB hole 204 at the top layer should be considerably large such that the FIB hole 204 reaches the bottom wiring layer 201-1. Since the FIB hole 204 having a large diameter has high possibility to affect the signal wirings, the FIB hole 204 cannot reach the bottom wiring layer (the power-source wiring) 201-1 and, as described above, it is very difficult to expose the bottom wiring layer 201-1. Therefore, it is difficult for a destructive inspection to measure a voltage fluctuation waveform of the power-source voltage of a LSI with a large number of wiring layers.
Meanwhile, since the current situation where an operating voltage is lowering causes decline of the absolute value of a fluctuation, it is difficult for the latter measurement method to measure an accurate voltage fluctuation waveform of a power-source voltage. Especially, if an increased number of wiring layers prolongs the distance between the position of a power-source wiring on the bottom layer and the observation terminal (the pad) on the top layer, the measurement for an accurate voltage fluctuation amount becomes further difficult.
As described above, an LSI having a large number of wiring layers and being operated on a lower voltage makes the conventional measurement methods difficult to measure a power-source voltage fluctuation of such a LSI so that the demand for comparison and verification between the results of voltage fluctuation waveforms obtained by a simulation performed as wiring design and by actual measurement performed on a final product LSI cannot fulfilled.